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Technischen Universität Berlin (2010)

Adigrat Sandstone in Northern and Central Ethiopia : Stratigraphy, Facies, Depositional Environments and Palynology

Enkurie, Dawit Lebenie

Titre : Adigrat Sandstone in Northern and Central Ethiopia : Stratigraphy, Facies, Depositional Environments and Palynology

Auteur : Enkurie, Dawit Lebenie

Université de soutenance : Technischen Universität Berlin

Grade : Doktor der Naturwissenschaften (Dr. rer. nat.) 2010

Résumé
In northern and central Ethiopia, continental to shallow-marine siliciclastic sediments of up to 430 m thick are exposed. These sediments are referred to as the ‘Adigrat Sandstone’. This study provides a detailed investigation of the stratigraphy, sedimentary facies, depositional environments and palynology of the ‘Adigrat Sandstone’ succession in the Mekelle and Blue Nile basins in order to obtain a complete picture of the large-scale spatial/temporal stacking patterns of depositional systems, whereby a new knowledge of the mode of evolution of the basins and the mechanisms controlling their formation can be obtained. The study also provides a more reliable stratigraphic position for the sandstone succession based on palynological data. Three unconformity bounded stratigraphic units have been identified within the Late Triassic to Middle Jurassic ‘Adigrat Sandstone’ succession. Unit I that represents the lower stratigraphic unit in both basins is Late Triassic (Late Carnian–Early Rhaetian) in age. It is composed of transgressive tide-dominated estuarine and prograding storm-dominated shoreface deposits. The overall northeastward-thickening wedge-shaped geometry of the depositional body and the palaeocurrent patterns in both basins suggest that the sediments accumulated on a vast slowly subsiding passive continental margin in a shallow gulf, which extended northeast into the Neotethys. This indicates that the Late Triassic shallow gulf, which encroached into the Arabian platform from the northeast, had most probably reached the northern and central parts of Ethiopia. A Neotethyan seaway through Saudi Arabia into Ethiopia has an important bearing on the palaeotectonic and palaeogeographic scenario of the region at that time. The second stratigraphic unit (Unit II) is Late Liassic (Early Toarcian) in age. No direct similarity can be inferred regarding facies successions and depositional systems within Unit II in the two basins. In the Mekelle Basin, the unit is composed of coarse-grained sandstones and conglomerates of fluvial and deltaic origin whereas in the Blue Nile Basin the unit is of barrier/inlet-spit origin. The fluvio-deltaic deposits in the Mekelle Basin have been accumulated in an active rift basin. The key similarity in the strata of Unit II in both basins is their similar palaeocurrent patterns, indicating deposition on a SE-dipping divergent continental margin. The abandonment of the previously existing NE-dipping continental margin and its replacement by a SE-dipping one can be best explained by rifting and basin inversion, which is most probably associated with the break-up of East and West Gondwana that commenced during the Toarcian. The uppermost stratigraphic unit (Unit III), which is recognised only in the Mekelle Basin, is Middle Jurassic (Late Toarcian to Early Callovian) in age. This phase corresponds to delta abandonment and the inception of transgressive barrier-lagoon system, which is most probably related to the continuous crustal thinning and subsidence along the evolving rift between East and West Gondwana. The transition from a ‘Gilbert-type’ delta to a barrier-lagoon system suggests that the basin has evolved from a possibly fault-controlled, broadly downwarping basin to a cratonic seaway characterised by a low gradient coastal to nearshore profile. The landward migration of the barrier-lagoon complex is attributed to the marine transgression progressing from the southeast as the rate of sea level rise exceeds the rate of sediment supply. As relative sea level started to fall, the elongate shoestring sand bodies of the uppermost barrier were destructed and cannibalised by a number of tidal inlet channels, which cut through them. Barrier destruction was concomitantly accompanied by the establishment of prograding open-coast tidal flats and strandplains. Based on 16 productive samples collected at different stratigraphic levels, three informal palynological assemblage zones have been identified within the Adigrat Sandstone succession in the Mekelle and Blue Nile basins. These assemblage zones, in ascending order, are : AZ I, Late Triassic (Late Carnian–Early Rhaetian) ; AZ II, latest Early Jurassic (Early Toarcian) ; and AZ III, latest Early–Middle Jurassic (Late Toarcian–Early Callovian). These palynological results allow for the first time a better biostratigraphic subdivision of the Adigrat Sandstone succession and its correlation with equivalent units in the region.

Mots Clés : Blue Nile Basin , Mekelte Basin , Basin evolution , Regional correlation , Paleogeography

Présentation

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Page publiée le 17 mars 2011, mise à jour le 4 novembre 2018